There are many known processes for producing silicons used as semiconductors and photovoltaic cell materials, and some processes are performed in the industry.
One of such processes is the so-called Siemens process, in which a silicon rod heated by energization to a silicon deposition temperature is placed in a bell jar, and trichlorosilane (SiHCl3) or monosilane (SiH4) together with a reducing gas such as hydrogen are brought into contact with the rod to deposit silicon.
This process provides high-purity silicon and is performed most commonly in the industry. Because of batchwise deposition, however, the process requires repeating a complicated procedure for every batch, including placement of the silicon rod as a seedbed, energization heating, deposition, cooling and takeout of the silicon rod, as well as bell jar washing.
Meanwhile, methods are proposed for continually producing polycrystalline silicon by use of an apparatus as shown in FIG. 12 herein (for example, JP-A-2003-2627 and JP-A-2002-29726). A silicon production apparatus 100 includes a closed vessel 111 equipped with a reaction tube 102, a gas supply opening 103 for supplying a chlorosilane and hydrogen, and a high frequency heating coil 104 provided around the outer periphery of the reaction tube 102.
The reaction tube 102 is heated with an electromagnetic wave from the high frequency heating coil 104 around the outer periphery thereof, so that the inner surface of the reaction tube 102 is brought to or above the melting point of silicon or to a lower temperature at which silicon can be deposited.
Subsequently, a chlorosilane from the gas supply opening 103 is allowed to contact with the inner surface of the heated reaction tube 102 to deposit silicon.
When the silicon deposition is performed while the inner surface of the reaction tube 102 has a temperature equal to or in excess of the melting point of silicon (first method), the silicon melt deposited in a molten state is allowed to continually drip down from an opening at a lower end portion 102a of the reaction tube 102 and is collected in a silicon collection part 105 provided underneath.
When the silicon deposition is performed while the inner surface of the reaction tube 102 has a temperature below the melting point at which silicon deposition is feasible (second method), solid silicon is temporarily deposited on the inner surface of the reaction tube 102 and the inner surface is heated to a temperature equal to or in excess of the silicon melting point, so that part or whole of the deposit is molten and is allowed to drip down into the silicon collection part 105 provided underneath.
In the reaction apparatus 100, regions in which the silicon deposition should be avoided, for example a gap 107 between the reaction tube 102 and a gas supply tube 106, are filled with a seal gas such as hydrogen. An exhaust gas from reaction in the reaction tube 102 is discharged through a gas discharge tube 108 in the closed vessel 111. The numeral 110 denotes a bulkhead wall made of quartz or the like that shields the high frequency heating coil 104 against the reaction gas atmosphere.
When a silicon deposition region of the inner surface of the reaction tube 102 is heated with the high frequency heating coil 104 to a temperature equal to or in excess of the melting point of silicon, a particularly great amount of heat is released from the lower end portion 102a of the reaction tube 102 and consequently the temperature becomes lower than that of the upper tube surface.
Accordingly, the following problem is encountered when the silicon deposition is performed at a temperature of the inner surface of the reaction tube 102 equal to or in excess of the silicon melting point (first method) or at a temperature below the melting point at which silicon deposition is feasible (second method). That is, when the inner surface of the reaction tube 102 is heated to a temperature equal to or in excess of the melting point of silicon to cause the deposited silicon to melt and drip down into the silicon collection part 105 provided underneath, the silicon melt dripping downward on the inner surface of the reaction tube 102 is cooled at the lower end portion 102a and is partially solidified.
The solidification of the silicon melt at the lower end portion 102a results in growth of a silicon mass hanging like an icicle from the tip of the lower end portion 102a. Thus, the silicon melt cannot be dropped and collected in the silicon collection part 105 appropriately.
If the lower end portion 102a is heated sufficiently with the high frequency heating coil 104 to prevent the silicon melt from solidifying at the lower end portion 102a, the portion above the lower end portion 102a is excessively heated. The results are that silicon fine powder and by-products such as silane oligomers are formed easily and objective silicon yield and energy loss are greatly deteriorated.
To prevent the heat release from the reaction tube 102, a thermal insulating member is often wound around the outer surface of the reaction tube 102. The existing thermal insulating members, however, cannot achieve sufficient thermal insulation of the lower end portion 102a. Furthermore, the silicon melt running on the lower end portion 102a from the inner tube surface to the outer tube surface through the opening, will contact with a lower end portion of the thermal insulating member to deteriorate the thermal insulating member.
The present invention has been made in order to solve the aforesaid problems of the conventional art. It is therefore an object of the invention to provide a silicon production apparatus whereby when deposited silicon is caused to drip down into an underlying collection part by heating the reaction tube inner surface at a temperature equal to or in excess of the melting point of silicon, the silicon melt can be prevented from solidifying at a lower end portion of the reaction tube due to temperature lowering at the lower end portion.